Polypropylene resins have high physical properties and moldability, and are an eco-friendly material. For this reason, use of the polypropylene resins has been rapidly spread over a wide range. Particularly, in automobile parts and the like, polypropylene resin products having a light weight and high rigidity are applied. One of such products is foam-injection-molded articles of polypropylene resin.
Examples of the polypropylene resin foamed articles include those obtained by melt mixing a mixture of a polypropylene resin and a foaming agent and extruding it under atmospheric pressure in an extruder to form an extrusion foamed sheet, and then sandwiching the sheet between molds to form a molded article; and those obtained by filling polypropylene resin pre-foamed particles into a mold having a desired shape and heating them by steam or the like to fuse the pre-foamed particles into a molded article. The polypropylene resin foamed articles are used in various applications because of their light weight.
Regarding the foam-injection-molded articles of polypropylene resin, examples of a technique of highly foaming a polypropylene resin include the so-called core-back method (moving cavity method) in which a resin containing a foaming agent is injected into a cavity of a mold held such that the mold can be opened, and the mold is opened to extend the cavity, thereby foaming the resin (for example, see Patent Document 1).
Usually, as properties of the polypropylene resin used for foam injection molding, fluidity for fully filling the resin into the mold and foaming properties for foaming the resin after filling are required. For example, in the case where the foam-injection-molded article of polypropylene resin constitutes, in particular, a large molded product as an automobile part such as a console box, a baggage box, a door trim, and a tool box, the molded article is required to be lightweight, thick, and highly rigid. In this case, the fluidity of a raw material resin to be used needs to be increased because molding defects such as short shots in molding are likely to occur.
Moreover, linear polypropylene resins usually used are crystalline and have small melt tension, which easily causes breakage of cells. Thus it is difficult to highly foam the linear polypropylene resin. As a result, appearance defects called silver streaks due to the gas produced by the foaming agent are likely to appear on the surface of the foam-injection-molded article, and voids are also likely to appear within the article. For this reason, it is difficult to increase the expansion ratio.
As a method for increasing the melt tension of polypropylene resins, for example, a method in which a crosslinking agent or a silane-grafted thermoplastic resin is added (Patent Documents 2 and 3), a method in which a non-crosslinked polypropylene resin is irradiated with radiation to introduce long-chain branches (Patent Document 4), and a method in which a polypropylene resin, isoprene monomer, and a radical polymerization initiator are melt kneaded to produce a modified polypropylene resin (Patent Document 5) have been proposed. To be sure, by these methods, a foam-injection-molded article with high expansion ratio is obtained; however, the viscosity during melting of the resin is excessively increased, leading to difficulties in injection molding. Moreover, flow marks, molding defects that are presumably attributed to imparting foaming properties, may appear, resulting in poor surface appearance of the article.
Meanwhile, as a method for improving moldability in foam injection molding, for example, a method has been proposed in which a linear polypropylene resin is used in combination with a polypropylene resin having a melt flow rate of less than 30 g/10 min and a melt tension of not less than 5 cN (Patent Documents 6 and 8). The method can improve the fluidity, and enables foam injection molding without short shots even when a relatively large mold is used. Nevertheless, this method may provide insufficient fluidity in the case of foam injection molding using a large mold such as those having a projected area of more than 0.2 m2.
Alternatively, a method using a polyolefin wax together has been proposed (Patent Document 9). This method further improves the fluidity; however, the use of wax may reduce physical properties (impact resistance) of the molded article. Particularly, in the case where a thin foam-injection-molded article is produced at a high expansion ratio of, for example, more than 2 times, an initial cavity clearance needs to be small, which may result in short shots in foam injection molding using a large mold. The melt tension is also largely influenced by the melt flow rate. The melt tension tends to be higher as the melt flow rate is lower. Accordingly, in a polypropylene resin having high fluidity in which the melt flow rate is, for example, more than 30 g/10 min, the specified value of the melt tension of not less than 5 cN is insufficient as an index of good foaming properties.
Meanwhile, for example, a method in which a polypropylene resin whose melt tension and loss tangent are specified is used for extrusion foaming (Patent Document 7) has been proposed. The resin used in this method has a low melt flow rate and is suitable for extrusion foaming, but not suitable for foam injection molding.
As described above, it is difficult to provide a polypropylene resin that is for use in foam injection molding using a large mold and has both better fluidity and better foaming properties, as well as measures against the appearance defects mentioned above.
Patent Document 1: WO 2005/026255
Patent Document 2: JP S61-152754 A
Patent Document 3: JP H07-109372 A
Patent Document 4: JP 2001-226510 A
Patent Document 5: JP H09-188774 A
Patent Document 6: WO 2009/060792
Patent Document 7: JP 2009-29900 A
Patent Document 8: WO 2005/026255
Patent Document 9: JP 2008-101060 A